This invention relates to an improved process for preparing the liquid irritant agent 1-methoxycycloheptatriene.
The invention further relates to an improved process for preparing the irritant agent 1-methoxycycloheptatriene from tropylium tetrafluoborate through use of a polymerization/condensation inhibitor to increase the yield and purity of the agent.
The invention still further relates to a more rapid and efficient method for isomerization of 7-methoxycycloheptatriene to its isomer 1-methoxycycloheptatriene.
The existing processes for preparing 1-methoxycycloheptatriene basically consists of a three step synthesis wherein an aqueous solution of cycloheptatrienyl tetrafluoroborate (tropylium tetrafluoroborate) in methanol is first converted to the 7-methoxycyclohepta 1,3,5-triene isomer by reaction with sodium bicarbonate; the resulting 7-isomer is thermally isomerized to 3-methoxycyclohepta-1,3,5-triene and this 3-isomer is in turn is either thermally isomerized to 1-methoxycyclohepta-1,3,5-triene or isomerized in a methanol solution, under acid catalysis, to the 1-methoxycyclohepta-1,3,5-triene isomer. The prior art process which had given the best yield of 1-methoxycylohepta-1,3,5-triene, i.e, approximately 43% with a purity of 85-89 mole % is best illustrated schematically in the following series of three reaction equations; ##STR1##
In the first step of the above synthesis, warm water is used to dissolve the cycloheptatrienyl tetrafluoborate (tropylium tetrafluoborate) (I) salt under a nitrogen atmosphere. Methanol is added to the resulting aqueous solution of (I), followed by the addition of solid sodium bicarbonate to give the reaction product 7-methoxycycloheptatriene (II). The 7-methoxycycloheptatriene (II) is isomerized by heating at 150.degree. C. for two hours to produce 3-methoxycycloheptatriene (III). In the third step, 3-methoxycycloheptatriene (III) is isomerized in an acid catalyzed rearrangement utilizing hydrogen chloride in methanol at room temperature for two hours, followed by neutralization with sodium bicarbonate, filtration and fractional distillation to give the product 1-methoxycycloheptatriene (IV).
The prior art processes described above have resulted from numerous studies of the isomerization reaction of alkoxycycloheptatriene. The production of cycloheptatrienylium bromide, its hydrogenation to cycloheptane, its subsequent conversion by phenyllithium to 7-phenylcycloheptatriene and the synthesis of 7-methoxycyclohepta (1,3,5) triene was first disclosed in The Cycloheptatrienyl (Tropylium) Ion., J. Am. Chem. Soc. 76, 3202 (1954) by W. Von E. Doering et al. Synthesis of 1-ethoxy-1,3,5-cycloheptatriene by reaction of 1-ethoxycyclohexene with dichlorocarbene and hot quinoline rearrangement is shown in A Synthesis of 3,5-Cycloheptadienone by W. E. Parham et al, J. Am. Chem. Soc. 84, 1755 (1962). Irradiation of 7-methoxycyclohepta-1,3,5-triene in the vapor phase has given a 40% yield of 1-methoxycyclohepta-1,3,5 triene by the process of O. L. Chapman et al, An Anomalous Photoisomerization in the Cycloheptatriene Series, Proc. Chem. Soc. at 221, July 1963.
Thermal isomerization of 7-methoxycycloheptatriene at 150.degree. C. by irreversible reaction into 3-methoxycycloheptatriene and subsequent reversible isomerization of the 3-isomer to 1-methoxycycloheptatriene under the same conditions is disclosed in Thermal Isomerization of Alkoxycycloheptatriene, Proc. Chem. Soc., February 1964, 59 by E. Weth et al, and A. Piter Borg et al, The Chemistry of Cycloheptatriene Part XII: The Thermal Behaviour of Substituted Cycloheptatrienes, Rec. Trav. Chim. 84, 1230 (1965). Thermal isomerization of alkoxycycloheptatrienes, e.g., 7-methoxycycloheptatriene in sealed tubes at temperatures in the range of 75.degree.-220.degree. C., i.e., under pressure to give a mixture of 3-methoxy-1-methoxy and 2-methoxy-cycloheptatriene is disclosed in T. Nozoe et al, The Thermal Isomerization of Alkoxycycloheptatrienes and Some Reactions of its Product, Bull Chem. Soc. of Japan, 38, 665-674, April 1965. T. Tezuka et al disclosed in Thermal Reactions of Alkoxycycloheptatriene at 300.degree.-800.degree. C., Chem. Lett. Japan, pp 1341-1346 (Chemical Society of Japan), 1974, that 7-methoxycycloheptatriene is completely consumed at 300.degree.-400.degree. C. to form 3-methoxy and 1 -methoxycycloheptatrienes when pyrolyzed by passing through a quartz column containing heated quartz tips. Heating at 450.degree. gave 1-methoxycycloheptatriene as the sole isomer product.
Finally, the prior art has recognized the benefit of excluding moisture from the thermal isomerization reaction of 7-methoxycycloheptatriene to the 3-methoxy and subsequently rearrangement to the 1-methoxy isomer by conducting the reaction at 150.degree. C. for 2.5 hr in a nitrogen atmosphere.
In all the above described methods of preparing the irritant 1-methoxycycloheptatriene, yields at each step are adversely effected by high-boiling polymeric or condensation-type material recovered as pot residue after distillation of the crude product. The overall yield of product and the purity of the product 1-isomer has been limited. Applicant's process has succeeded in obtaining significantly better yields of higher purity product through elimination of the adverse effects of these high boiling polymeric materials.